Preparation, Structure And Properties Of Porous Silk Fibroin Films

The fibroin solids (film and gel) were prepared from regenerated silk fibroin solution in different ways. The structural and structural changes of the fibroin solids during storage were studied. The results indicated that the fibroin film air-dried at 30℃or freeze dried above -20℃was the coexistence of amorphous and silkⅠstructure. Part of the amorphous could be transformed into silkⅠstructure under room temperature and humidity. The structure of porous silk fibroin film was the coexistence of amorphous and small part of silkⅡ, if the aqueous fibroin was freeze dried within the temperature range of -80℃to -20℃, and the amorphous partly transformed into silkⅠduring storage. The structure of fibroin gel naturally forming when kept at room temperature was coexistent of amorphous and silkⅡ. The content of Gly and Ala was high in the surface film of aqueous fibroin and its structure was mostly silkⅠand silkⅡwith high crystallinity.The effects of the amount of added epoxide resin and its molecular weight were investigated on the structure and properties of silk fibroin film crosslinked by poly(ether epoxide) resin. It was indicated that the weight loss rate in hot water of fibroin film and the initial tensile modulus decreased, the elongation rate at break and the tensile elasticity increased as the added epoxide resin was increased. The added epoxide resin could notably decrease the solubility of fibroin film and improve its mechanical properties. As the added catalyst was increased within a certain range, the properties of crosslinked fibroin film improved to some degree.Silk fibroin films by adding polyhydric alcohol or exerting reinforced material were prepared, and the film properties were measured such as strenth, elongation, solubility, water permeability etc. It is found that the low polyhydric alcohol cross-linking with fibroin can improve the mechanical properties of film, and exert reinforced material can make high strength silk fibroin film.The state of frozen silk fibroin solution and the fine structure of freeze-dried porous silk fibroin materials were tested and analyzed. The results indicated that the glass transition temperature of frozen silk fibroin solution ranges from -34℃to -20℃, and the initial melting temperature of ice in frozen solution is about -8.5℃. When porous silk fibroin materials are prepared by means of freeze-drying, if freezing temperature is below -20℃. the structure of silk fibroin is mainly amorphous with a little silkⅡcrystal structure, and if freezing temperature is above -20℃, quite a lot of silkⅠcrystal structure forms.Porous silk fibroin films, with average pore size 10～300μm, pore density 1～2000/mm~2 and porosity 35～70%, were prepared by freeze-drying aqueous solution of silk fibroin. Pore size distribution of such materials mostly accorded with logarithmic normal distribution. It is possible to control the aforementioned structural parameters and the physical properties as moisture permeability, compressibility, strength and elongation etc., by adjusting freezing temperature and concentration of silk fibroin solution. Above glass transition zone (-34～-20℃) of silk fibroin, the freezing temperature has more significant effect on the structure and properties of porous silk fibroin materials.The influence of repeated freeze-thawing on pore structural characteristics and physical properties of porous silk fibroin materials prepared by freeze drying were studied. It showed that when quick-frozen silk fibroin solution was repeatedly thawed and frozen before being vacuum dried, pore size of thus prepared porous silk fibroin materials increased from 67μm to about 120μm, and pore density decreased from 80 1/mm~2 to about 28 1/mm~2; at the same time compression ratio and moisture permeability increased from 22.7% and 230 g/m~2h to about 33.7% and 308 g/m~2h respectively, tensile strength and dissolvability in hot water decreased from 20.2 N/cm~2 and 42.7% to about 12.5 N/cm~2 and 26.1% respectively. Both the times of repeated thawing and the thawing temperature had certain influence on the above-mentioned pore characteristic parameters and physical properties.The relations of the blend ratio and freezing temperature with the morphology, fine structure and properties of freeze-dried porous PVA/SF films were investigated. It was indicated that the pore diameter of the films decreased and the pore density increased as the increase of the proportion of PVA, and as the freezing temperature increased, things were reverse. By adjusting the blend ratio and the freezing temperature, various kinds of PVA/SF films with different pore structural parameters and mechanical properties could be prepared. The blend ratio affects the compatibility of the PVA/SF films, the lower the proportion of PVA, the better the compatibility. According to the experimental results in this research, the improvement of the compatibility of PVA/SF did not make the mechanical properties better than that of cross-linked porous SF films.SF/PVA gel was prepared from frozen silk fibroin (SF) and poly (vinyl alcohol) (PVA) solution. The effects of freezing temperature, the times of repeated freezing and the drying method on the structure and properties of the gel were studied. The results indicated that the strength and elasticity of SF/PVA gel increased as the decrease of freezing temperature and the increase of freezing times. The crystallinity, strength and elasticity of elongation of air-dried SF/PVA gel at 25℃were relatively high. By repeated freezing, the pore size and porosity within SF/PVA gel increased slightly and the pore density decreased a little.The porous protein blend film of gelatin and domestic silk fibroin was prepared by freeze-drying. The effects of the cross-linking agent, blend ratio and freeze temperature on the structure and properties of film were investigated. The results indicated that the porous blend film was cross-linked obviously as a function of glutaraldehyde, silk fibroin and gelatin in the film were compatible and the structure of protein was mainly amorphous, and that the tensility of porous silk fibroin film was improved by blending with gelatin.Degradation behavior of silk fibroin sheet was investigated by enzymatic degradation experiments in vitro with ProtenaseⅩⅣ, CollagenaseⅠA andα-Chymotrypsin respectively. The results indicated that all the three enzymes could degrade silk fibroin sheet into amino acid and peptide, and that the degradation speed of ProtenaseⅩⅣwas the quickest, CollagenaseⅠA the second andα-Chymotrypsin the slowest, and that the average molecular weight of degradation products with the three enzymes was just in reverse order. ProtenaseⅩⅣhad strong ability of degrading silk fibroin sheet. 70 percent of silk fibroin sheet could be degraded in 15 days at 37℃in 1.0 unit/ml ProtenaseⅩⅣsolution, and more than 50 percent of the degradation products were free amino acid. In the first 9 days for degrading porous silk fibroin sheets with ProtenaseⅩⅣand CollagenaseⅠA, the pore size inside the sheets increased slightly. After 9 days, silk fibroin sheets collapsed and became shapeless totally.The porous silk fibroin film was proved to be no acute toxicity or irritation or allergy after the test on quite a few indices in more than a month. It was also confirmed that human keratinocytes could be cultivated in porous fibroin film of pore size of 100μm. The animal test showed that newly grown blood vessel and fibroblast cell could grow in the film.